Railway trucks are well known in the railway industry and it has been common practice to support the opposite ends of a freight car body on a pair of spaced car trucks. Each truck comprises two wheel sets mounted on axles, with both axles being joined by and supported by a pair of spaced side frame casting members which extend generally longitudinally along the opposite sides of the car body. The side frames are located outboard of the wheels and are mounted on the axles by roller bearing assemblies with appropriate adapters. An elongated bolster casting is centrally mounted parallel to the axles and received within a window in each of the side frames castings. The bolster casting is supported within each respective side frame casting by a suspension system including respective spring sets on each bolster end for permitting limited movement of the bolster relative to the side frames. Depending upon the load capacity of the railcar, the spring set can comprise a varying number of outer coils, inner coils, or shock absorbing devices. In the various spring set configurations, the springs extend between a spring seat on each side frame and a respective undersurface of the bolster, holding the bolster in a spaced relationship relative to the spring seat. The weight of the freight car body is generally supported by a centrally disposed bolster center plate, but when the car body laterally tips, some weight is then transferred to either of a pair of bolster-mounted side bearing assemblies. Each side bearing assembly is located generally on the distal bolster end and inboard of its respective side frame. The bolster center plate is centrally disposed between each of the bearing assemblies. Typically, there are four major types of car instability that are directly related to this type of freight car body support and they will now be described.
The first type of car instability is referred to as truck hunting, which is caused by lateral forces imputed to the car body. Hunting usually occurs at high speeds wherein the truck assembly no longer remains parallel to the rails, causing it to weave down the track, usually with the wheel flanges striking the rails. In addition, truck lozenging or warping accompanies such hunting wherein the bolster turns out of square with respect to the side frames.
The second type of instability is referred to as rock and roll and this type of car instability usually occurs at low speeds and is caused by the joints in the tracks. Jointed track is frequently non-planar due to excessive settlement which results from worn joints and non-uniform ballast or foundation under the railway ties. Because track joints are staggered with respect to the rail pairs, a railcar will first experience a joint on one rail before experiencing the next successive joint on the opposite rail; the alternating pattern continues as the car travels down the track. Each time the unplanar rail ends forming the joint are encountered, the wheel movements in the truck assemblies will impart energy to the truck suspension system, causing the car body to rock or sway excessively in a lateral direction with respect to the tracks.
The third type of instability is caused by bouncing or pitching of the car body when the railcar experiences a dip or rise in the track. This instability occurs in a direction which coincides with the length of the railcar.
The final type of car instability, which the present invention addresses, is similar to hunting, in that it is another form of lateral instability. It is typically excited by track irregularities, such as worn track joints, wherein lateral acceleration is being transmitted into the car body. This type of car instability also has a linear relationship with respect to car speed, meaning that as the speed of the train increases, the car will become increasingly unstable, especially at high speeds (60 mph and above). Like hunting, any lateral instability imputed to the car body from this form of instability will correspondingly decrease the speed at which the car can be safely operated. Therefore, it is a common desire of railroad operators to eliminate as many types of car instability as possible. When specifically trying to reduce or eliminate the fourth type of instability just described, it has been discovered that if the car body can be isolated or decoupled from the truck assembly (including the bolster), the lateral acceleration or lateral motion on the car body can be effectively controlled.
There has been considerable prior art describing the physical decoupling of the lateral forces between the track and the car body. For example, in the passenger car field, swing hangers have been used for years, where the approach is to suspend the car with links that permit the car body to swivel with respect to the wheelsets. However, the biggest disadvantage of swing hangers is that they are very expensive and therefore impractical for use in the freight car field.
Other methods for isolating the lateral forces involved the incorporation of various means for decoupling the truck from the car body at the axle or journal. In general, this art fell into three categories: 1) Plain bearings, which were comprised of brass shells lined with a babbitt material, thereby providing considerable lateral travel with well lubricated surfaces; 2) Cylindrical roller bearings, which allowed the axle to slide relative to the truck, thus decoupling the lateral motions from the car body; and 3) Sliding bearing adapters, which placed an elastomeric pad between the bearing adapter and the sideframe, allowing lateral motion to be isolated before being transferred into the bolster. However, there has not been much developed art which describes a friction shoe sliding with respect to the bolster as the means for decoupling the lateral motions of the truck from the car body.
The most common means employed today for dissipating the energies imparted to the truck assembly suspension system use friction shoe assemblies which dampen the relative vertical motion between the bolster and the side frames. Typically, each truck bolster end includes a pair of opposed friction shoe pockets, each of which houses either a single or double friction shoe. Each pocket includes a pair of spaced, sloped surfaces which a engage a corresponding pair of sloped surfaces on the friction shoe, thereby transferring a load imposed by a steel coil biasing spring, placed below the friction shoe, from a vertical to a horizontal orientation. Each friction shoe also includes a flat, vertical face which is in sliding frictional engagement with a replaceable hardened steel frictional wear plate attached to each of the bolster side frame columns, thereby frictionally dissipating any imparted energy.
In this respect, improvements made to friction shoe devices have mainly concentrated on improving the characteristics of the shoe when experiencing vertically directed forces. Since the magnitude of the vertical forces acting upon the bolster are far greater than the magnitude of even the largest lateral forces which would ever act upon the railcar, the spring coil groups and the friction shoe springs of previous suspension systems were designed for addressing the vertical forces. Furthermore, since those magnitudes were so much larger, the frictional interface between any given surface on the friction shoe and the friction shoe pocket walls was so great that the friction shoe could not laterally move within the friction shoe pocket and decouple the lateral forces directed to the railcar. Since lateral movement was naturally stifled, friction shoe pockets were designed with little or no tolerance between the shoe and the pocket.
However, U.S. Pat. No. 4,167,907, developed a three piece friction shoe which was said to allow more lateral decoupling than previous art friction shoes. This shoe was limited to variable control spring loading applications, where the shoe spring deflection is directly related to the amount of vertical bolster deflection. In that design, the lateral decoupling movement which was said to exist, was truly illusory because lateral movement in a variable control spring design will only be possible when the lateral forces are greater than the vertical forces; arguably this situation might have merit but only when the cars are traveling empty.
On the otherhand, in a constant control spring system, where the shoe spring is interposed between the shoe and the base of the friction shoe pocket, downward bolster deflections are not directly experienced by the friction shoe spring. This means that with the constant control system, overcoming lateral decoupling forces becomes a matter of overcoming the static friction forces between the friction shoe and the friction shoe pocket surfaces. It should be understood that in a constant control application, the static friction forces are very small when compared to the vertical loading forces which have to be overcome in a variable control application before lateral movement is imparted. However, it should also be understood that it is particularly important to overcome these static friction forces before they approach magnitudes which could exceed the bending stiffness of the control spring, otherwise, the spring will buckle sideways, causing the shoe to become jammed within the shoe pocket.